Sputtering is the physical ejection of material from a target as a result of ion bombardment of the target. The ions are usually created by collisions between gas atoms and electrons in a glow discharge. The ions are accelerated into the target cathode by an electric field. A substrate is placed in a suitable location so that it intercepts a portion of the ejected atoms. Thus, a coating of target material is deposited on the surface of the substrate.
In an endeavor to attain increased deposition rates, magnetically enhanced targets have been used. In a planar magnetron, the cathode includes an array of permanent magnets arranged in a closed loop and mounted in a fixed position in relation to the flat target plate. Thus, the magnetic field is caused to travel in a closed loop, commonly referred to as a "race track", which establishes the path or region along which sputtering or erosion of the target material takes place. In a magnetron cathode, a magnetic field confines the glow discharge plasma and increases the path length of the electrons moving under the influence of the electric field. This results in an increase in the gas atom-electron collision probability. This leads to a much higher sputtering rate than that obtained without the use of magnetic confinement. Further, the sputtering process can be accomplished at a much lower gas pressure.
In dc reactive sputtering, a reactant gas forms a compound with the material which is sputtered from the target plate. When the target plate is silicon, and the reactive gas is oxygen, silicon dioxide is formed on the surface of the substrate. However, because silicon dioxide is such a good insulator, a film thick enough to cause arcing is rapidly formed in areas of the target plate outside of the race track. Silicon dioxide is known to be one of the most difficult dielectric films to deposit by magnetron reactive sputtering because of this characteristic. The arcing associated with silicon dioxide has prevented planar magnetron reactive sputtering from being efficiently utilized to deposit quality silicon dioxide films.
Another technique for coating silicon based compounds onto substrates involves reactive sputtering with a cylindrical magnetron having a silicon target. See Wolfe et al., U.S. Pat. No. 5,047,131, issued Sep. 10, 1991. In operation, due to the accumulation of dielectric material in various parts of the coating chamber, it is necessary to clean the system on a regular basis. Indeed, when coating silicon dioxide or silicon nitride by reactive sputtering, the system can operate continuously only for approximately 30 hours.
Finally, another limitation to the utility of both planar and cylindrical magnetrons (in either reactive or non-reactive sputtering) is that films deposited by sputtering have not achieved the degree of uniformity required for many high precision applications. This is true even during the initial 30 hours of sputtering. Recent attempts to improve film uniformity have been unsuccessful. See Dickey et al., U.S. Pat. No. 5,106,474, issued Apr. 21, 1992; Meyer, U.S. Pat. No. 4,849,087, issued Jul. 18, 1989; and Gillery et al., U.S. Pat. No. 4,478,702, issued Oct. 23, 1984.